Today, printed circuit boards (“PCBs”) are designed using computer aided design (“CAD”) tools. These CAD tools allow the designer to plan a circuit board design, including components mounted thereon and the interconnecting conductor traces, within a computer environment. Once a design is finalized after testing and optimization, the CAD tool outputs an image of the design. This image is used to etch the conductor traces onto the surface of the PCB.
PCBs are used as a substrate to support electronic components mounted thereon (e.g., surface mount components) and the conductor traces that interconnect these electronic components. Typical PCBs are constructed with an epoxy resin reinforced with a single layer of woven fiberglass fabric suspended therein. This epoxy resin and fiberglass composite forms a rigid insulating dielectric medium less than ideal for depositing electronic components and conductor traces thereon.
One drawback of a composite PCB is localized variations in its dielectric constant. The constituents of a composite PCB—epoxy resin and fiberglass fabric—have differing dielectric constants. When an electric field passes through a dielectric medium, it causes microscope distortions in its charge distribution by inducing dipoles and polarizing bound charges. These distortions in turn affect other electric properties of the dielectric medium. These distortions in a PCB are generally undesirable, but are particularly undesirable if not uniform throughout the PCB.
Since the typical PCB discussed above is a composite of epoxy resin and fiberglass fabric having differing dielectric constants, localized dielectric constants vary throughout the PCB. Thus, electric signals transmitted on conductor traces running across the surface of a composite PCB will experience varying effective dielectric constants. The cumulated effect of this varying dielectric constant results in detrimental timing skew. Thus, if two conductor traces running side by side are conducting signals across the PCB, the varying dielectric constant may cause one signal to arrive earlier than the other. When the receipt of these signals is time critical, this timing skew will deleteriously constrain component clock speeds and bandwidth.
Differences between the effective dielectric constant experienced by adjacent conductor traces running over an epoxy resin and fiberglass fabric composite have been measured as high as 0.3, which can limit conductor traces of 6–10 inches to a maximum frequency of 5–10 GHz. Currently, the problem of varying effective dielectric constants is addressed a number of different ways, including: adding multiple fiberglass fabric and resin layers to even out localized variations in the dielectric constant, shrinking feature sizes to limit conductor trace exposure to localized variations in the dielectric constant, and/or utilization of specialty composite materials having less divergent dielectric constants. However, all of these methods increase design complexity, fabrication costs, or both.